The upgrades vary. I generally book Flex and get upgrades if there are a reasonable amount of Premium seats available. The front line airport staff generally don’t know about the upgrade tool and whatever rules it’s currently using - that’s kind of a back office thing.

Interesting note, I booked Flex this week and changed my flight later (which is why I book Flex) and there were multiple Premium seats available but I didn’t get an upgrade on either leg. Not a big deal but I was wondering if I confused the poor little algorithm.